1. Field of the Invention
The present invention relates to: a toner set containing a chromatic toner and a transparent toner; a developer set using the toner set; and an image forming apparatus.
2. Description of the Related Art
In recent years, there is an increased demand for image forming apparatuses that can save energy for fixing toner and respond to high-speed processing, and toners themselves are required to melt at low temperatures. There is also an increased demand for attaining high-image quality. For high-quality image formation such as formation of photo images, gloss is imparted to a surface of recording media to provide vivid high-gloss images. For example, proposed are a method in which a transparent toner is applied to a non-image portion without a chromatic toner to thereby reduce the difference in gloss between an image portion with a chromatic toner and the non-image portion; and a method in which a transparent toner is applied to the entire surface of a recording medium (see Japanese Patent Application Laid-Open (JP-A) Nos. 04-278967, 04-362960 and 09-200551). Also, proposed is an apparatus including a fixing unit configured to heat a recording medium having a chromatic toner image and a transparent toner image for melting, followed by cooling and peeling off, to thereby form a high-gloss image on the recording medium (see JP-A No. 05-158364). According to these proposals, it is possible to reduce the difference in gloss over the entire surface of a recording medium to form an image having a uniform gloss.
In the field of printing, in order to control the gloss of a recording medium, UV varnish printing, varnishing or polypropylene (PP) laminating is generally employed to perform so-called spot varnish printing, which increases the gloss of a specific portion of a recording medium.
In such spot varnish printing, a plate for partially increasing the gloss after usual color printing is formed and coated with, for example, UV varnish for printing. A portion that has been subjected to the spot varnish printing can have a high gloss like a photo image. Meanwhile, the other portion that has not been subjected to the spot varnish printing has a low gloss. In this manner, the difference in gloss on the resultant image becomes large, so that these portions can be distinguished from each other to a more extent than in usual printing. However, the spot varnish printing in offset printing requires a special plate. In addition, offset printing cannot respond to variable data and thus requires a printing lot number more than a certain number. If electrophotographic image forming apparatuses such as laser printers and dry-process electrostatic copiers can realize a function of spot varnish printing, a plate for printing is not necessary, and also it is possible to respond to variable data.
Proposed methods for forming portions having different glosses on the same recording medium by an electrophotography process include: a method of controlling gloss by adjusting the number average molecular weight of a resin used in a toner (see JP-A No. 08-220821); a method in which a chromatic toner image is fixed and a transparent toner image is formed thereon, and then the fixing temperature is reduced to reduce gloss (see JP-A No. 2009-109926); and a method in which a first printing operation is performed on a gloss portion, followed by fixing, and a second printing operation is performed on a non-gloss portion, followed by fixing (see JP-A No. 04-338984). These proposals can form portions having different glosses on the same recording medium, but have not realized yet a high-gloss image comparable to a photo image obtained by the spot varnish printing.
As described above, various methods of controlling gloss on recording media using a transparent toner have been proposed. For example, JP-A No. 08-220821 describes a transparent toner containing a polyester resin having a number average molecular weight of about 3,500, and a chromatic toner containing a polyester resin having a number average molecular weight of about 10,000, where the melting point of the transparent toner is lower than that of chromatic toner, which improves smoothness and partially increases glossiness of the transparent toner portion. Meanwhile, since the transparent toner is formed on the uppermost layer of an image and comes into direct contact with a fixing unit, the transparent toner is required to have higher hot offset resistance than the chromatic toner. In addition, a layer of the toner becomes thick since the transparent toner layer is formed on the chromatic toner image. Moreover, unless the chromatic toner has high cold offset property, the resultant image formed from the combination of the transparent toner having a low melting point and the chromatic toner having a high melting point lacks stability. Therefore, in order to form a high-gloss image comparable to the aforementioned photo image, only adjusting the number average molecular weights of the resins used in the transparent and chromatic toners is not sufficient.
In the image forming method described in JP-A No. 2009-109926, the maximum melt viscosity of toner in the fixing nip in second image formation is set to be higher than that of toner in the fixing nip in first image formation. Thus, a transparent toner image formed in the second image formation does not sufficiently melt to result in reduction of gloss, making it impossible to form a high-gloss image comparable to a photo image.
JP-A No. 04-338984 describes that styrene-acryl copolymers and thermosetting resins and thermoplastic resins such as polyester resins can be used as a resin of a transparent toner, but does not disclose a constitution of toner for obtaining gloss.
Therefore, demand has arisen for provision or a toner set containing both a transparent toner and a chromatic toner, and being excellent in hot offset resistance and capable of attaining high glossiness and sufficient fixability at the same time; and an image forming apparatus capable of forming a color image using the toner set so as to make an intended region of the color image have different gloss from the other region thereof.